The present invention relates to a signal transmission system having a photo star coupler in a part of the system and, more particularly, to a signal transmission system which is suitable to block an echo back from the photo star coupler.
When a local network is constructed by a coaxial cable, a cable length of about 500 m is a limit value of communication. Therefore, a method of expanding the network by using an optical fiber has widely been used. As practical means of realizing such a method, there have been known a method whereby a plurality of local networks constructed by coaxial cables are coupled by optical fibers and a method whereby the coaxial cable is replaced by an optical fiber.
In the network for executing the bidirectional communication, not only a bus of the local network but also a connecting portion between buses of the local networks need a bidirectional communication path. In this case, when using the former expanding method (a plurality of local networks constructed by coaxial cables are connected by optical fibers), the local networks are not merely coupled by bidirectional optical fiber transmission paths, but they are coupled by using photo star couplers (hereinafter, simply referred to as star couplers). Three or more local networks can be coupled, so that it is advantageous in terms of preparation of the future expansion. On the other hand, when using the latter expanding method (the local networks are coupled by optical fibers in place of the coaxial cables and each terminal apparatus is connected to the optical fiber), a star coupler is used to form a bidirectional communication path between the terminal apparatuses.
The star coupler has a plurality of terminals and the terminals are all connected in the coupler. Therefore, when a photo signal is input to either one of the terminals, the photo input signal is distributed to the other terminals and an output. When a network is constructed by using such a star coupler and a bidirectional communication path is formed, a transmission fault called an echo back occurs.
That is, in the case where a first local network is connected between the first and second terminals of the star coupler and a second local network or a terminal apparatus is connected to the other terminal of the star coupler, a signal is sent from the first local network to the first terminal of the star coupler. There is a problem such that the system is influenced in a manner such that a transmission signal in the first local network is destroyed by a return signal from the second terminal of the star coupler or the return signal is again circulated.
A control to eliminate such an echo back is called an echo back inhibition control. Conventional examples in which such an inhibition control is executed have been disclosed in JP-A-62-157434 and JP-A-60-260246. According to the above prior arts, a repeater for executing a bidirectional communication is connected between the first and second terminals of a star coupler and a first local network, and in the repeater, a signal which is sent in the second direction opposite to the first direction of the star coupler is blocked for a period of time when the transmission signal in the first direction is detected and for a predetermined period of time after the time point when the transmission signal in the first direction is not detected. It is now considered that the signal which is sent in the second direction for such a period of time is called an echo back signal. By blocking such a signal, the transmission fault can be prevented.
In the above prior arts, the predetermined period of time from the time point when the transmission signal in the first direction is not detected is equal to the time which is required to reciprocate a signal between the repeater and the star coupler and such a time differs depending on a fiber length of each path. Therefore, each time the network construction changes and its fiber length changes due to expansion or the like, the delay time (the time which is required for signal reciprocation) must be again measured and the time must be set, so that is troublesome. As a countermeasure, it is considered that the set time is set to a slightly large constant value. However, if a terminal apparatus which had received the transmission signal transmitted the signal by normally responding during such a set time, the repeater blocks the signal, because it is undesirable. Further, as a countermeasure for such a problem, there is also a method whereby the delay time is adjusted by adding an additional fiber, thereby preventing that the terminal apparatus that normally responds. However, according to the above methods in which the set time is set to a value which is larger than it is needed, since the delay time of the frame is unnecessarily set to a large value, there is a problem such that the response speed deteriorates.